Sometime this error occurs that says:

"Unable to monitor file system. Please run: echo 100000 | sudo tee /proc/sys/fs/inotify/max_user_watches and restart Dropbox to correct the problem.

We need to adjust the system setting on the maximum file number that Dropbox can watch.

The following command will solve your pain:

$ echo fs.inotify.max_user_watches=100000 | sudo tee -a /etc/sysctl.conf; sudo sysctl -p

Here is the tip from Dropbox website.